Autosampler for a High-Payload Centrifuge
Winter 2013 Status Report
Senior Design 2012 – 2013
California State University, Los Angeles
March 22, 2013
Student Team Members:
Ye Ding, Crusberto Gonzalez, Yun S. Wang, and Yao Yuan
Advisors:
Yola Wong K, Prof. Stephen Felszeghy, Prof. Gustavo Menezes, and Prof. Arturo Pacheco-Vega
Abstract
During this quarter, the student team has refined the CAD models for the autosampler
probe sets, actuators, vacuum subsystem, and overall layout. Candidates for the actuators,
vacuum pumps, liquid pumps, microcontroller, and wireless transceivers are ready for
final approval by advisors. A significant amount of work remains, however, including the
verification of Coriolis force calculations, certain component prototyping and testing, and
electrical integration. By the most optimistic projection, a complete critical design review
must be postponed until the end of Spring quarter 2013.
Status Report Winter 2013
I. Introduction and Background
During the previous quarter of Fall 2012, preliminary analyses were performed to clarify project
requirements, estimate its scope, brainstorm ideas for the soilbox configuration, and pick out candidates
for the strain gauges and the soil moisture sampler probes. Progresses this quarter were made in the areas
of mass and size optimization, means of actuation, probe set stiffness, pump considerations, sample
trapping and storage, and wireless communication. More analyses and risk-reduction tests await, but a
clearer picture of how the project will look like has been formed (figure 1).
Figure 1. Overview of the centrifuge and the onboard payload systems minus tubing and electrical
connections. Two equivalent payloads will eventually be mounted on the centrifuge arm, one on each end, for
balance, but a dummy box of the same size and mass will be used at first to test the first payload system and before its
functional counterpart becomes ready. Major components inside each payload have been modeled, some with
candidates already narrowed down for final approval. All have been sized to fit the available space and are pending
further optimization to reduce total mass.
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Status Report Winter 2013
II. Technical Progress
Two of the most challenging constraints have been the maximum mass and size. As much unused space as
possible has been exploited so that everything can fit inside the encapsulating payload box. On the other
hand, the current total mass exceeds the project allowance and must be reduced further. In terms of
component design and selection, focuses have been placed on the ability to accommodate stress and
potential displacements caused by the centrifugal acceleration field and Coriolis effects, as well as the
minimization of potential sources of signal noise. Further analyses and testing are needed to determine
how water disperses through the soil while the centrifuge is spinning, the suction rate provided by the
pumps at various settings, levels of water saturation, and different types of soils, and the pressure drop
incurred by the hydrophobic barriers ideally to be incorporated inside the vacuum subsystem.
Mass, Power Budgets and Overall Layout
The current mass is 46.7 Kg over allowance, beyond which the centrifuge arm is not rated at the desired
speeds of rotation. The inherent factor of safety built into the centrifuge is unknown as of this date. For
each payload, the bulk of its mass is contributed by the water, soil, actuators, and external casing (table 1).
The research team is willing to lower the amount of water by 5 Kg; the rest of any mass savings must come
from the other components. Several kilograms may be shaved off of the combined mass of the actuators,
motors, soilbox, and load platform, but any significant reduction will probably have to come from a lighter
external casing. If worst comes to worst, the top speed of rotation must be reduced, lowering the
maximum centripetal acceleration field available for running experiments.
Since the power for running the centrifuge as well as the onboard payloads comes from the mains power
available on site, in this case, the basement of the engineering building, it may be less constrained than
the mass is (table 2). For the systems onboard each payload, the total power comes out to be 767.8 Watts.
The entire power consumption--peak and continuous--of both the payloads and the centrifuge itself may
have to be cleared with the facilities first, however, although considering the comparatively massive size
and speed of the centrifuge, the additional power required by our project may not be as substantial. As
with the mass budget, the contingency rating for each component is based on the maturity of the
technology and the risks to implement it. In addition, an overall 10% margin is given to the project as
most of the unknowns concerning mass and power have been eliminated.
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Status Report Winter 2013
Table 1. Project mass budget. Contingency ratings are based on technological maturity and implementation risks
(table 3).
Unit mass
Item total
(Kg)
(Kg)
2
0.5
1.0
5%
1.05
Source tank
1
2.0
2.0
10%
2.20
Contaminant tank
1
0.75
0.75
10%
0.825
Effluent tank
1
2.75
2.75
10%
3.025
Soilbox
1
6.0
6.0
10%
6.60
Liquid pump
3
0.5
1.5
5%
1.575
Load platform
1
3.0
3.0
10%
3.30
Dispersion cap
1
0.5
0.5
15%
0.575
Collection cap
1
0.5
0.5
10%
0.55
Actuator & motor
2
15.0
30.0
5%
31.5
Actuator attachments
2
1.0
2.0
10%
2.20
External casing
1
30.0
30.0
10%
33.0
Water
1
20.0
20.0
1%
20.2
Saturated soil
1
16.0
16.0
10%
17.6
Arduino MEGA
1
0.04
0.04
5%
0.0420
XBee shield
1
0.03
0.03
5%
0.0315
XBee chipset
1
0.02
0.02
5%
0.0210
Item
Quantity
Vacuum pump
Contingency
Item +
contingency (Kg)
Subtotal
124.3
10% Margin
12.43
Total mass
136.7
Max. allowance
90.00
Surplus
(-46.7)
Table 2. Project power budget. Contingency ratings are based on technological maturity and implementation
risks (table 3).
Unit peak
Item total
power (W)
(W)
2
300.0
600.0
5%
630.0
Liquid pump
3
30.0
60.0
5%
63.0
Vacuum pump
2
1.60
3.20
5%
3.36
Item
Quantity
Actuator & motor
Contingency
Item +
contingency (W)
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Status Report Winter 2013
Load-cell
2
0.30
0.60
5%
0.63
Arduino
1
0.20
0.20
5%
0.21
XBee chipset
1
0.10
0.10
5%
0.105
XBee shield
1
0.10
0.10
5%
0.105
Flow rate sensors
2
0.30
0.60
5%
0.63
Subtotal
698.04
10% Margin
69.8
Total peak power
767.84
Max. allowance
TBD
Surplus
TBD
Table 3. Contingency allocations for components of varying technological maturity and risks to
implement.
Component maturity and risks
Contingency allocation
New units containing new technologies
25%
New units based on existing technology
20%
Major modifications to existing units
15%
Minor modifications to existing units
10%
New unit with engineering models
10%
Off the shelf, qualified units
5%
Actual Measured power of unit
1%
The overall layout is influenced primarily by the need to maximize the utilization of all available space,
which is capped at 88 liters (figure 2). The most drastic changes from our first design concepts as a result
of spatial constraints are the reduction in the number of probe sets from four to two and the retention of
movement along only the vertical direction--an additional horizontal axis of motion were considered but
eventually rejected (figure 3). There have been impacts on the selection and orientation of the actuators as
well. The second foremost concern is the pressure developed inside the fluid piping system under the 100
G centripetal acceleration as the centrifuge spins up to the highest speed intended for research
experiments. Out of this consideration, the entire effluent collection tank has been placed downstream of
the soilbox in the acceleration field in order to eliminate the need for an additional pump on each of the
nine lines entering its respective chamber inside the collection tank (figure 4). Meanwhile, the remaining
two liquid pumps are positioned below their respective source tanks as well--either water or contaminant-
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Status Report Winter 2013
-so that their capabilities can better match the changing pressures developed as the initially full source
tanks are pumped dry (figure 4).
Figure 2. Latest CAD model of the payload showing components and their positions (external casing
not shown). Notably the number of probe sets has been reduced to two, and the pair of actuators is mounted to the
ceiling of the payload box and oriented in-line with the direction of fluid flow through the soilbox and along the
centrifugal acceleration developed when conducting actual experiments.
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Status Report Winter 2013
Figure 3. One of our first design concepts for the autosampler and soilbox assembly. Originally four
probe sets were planned with movement in the vertical direction. The two pairs of actuators would straddle the
soilbox from the sides. An additional horizontal degree of freedom was to be built into either the actuator assembly or
the platform supporting the soilbox; shown here is the box-on-rail concept. Notice also that the strain gauges were
mounted behind the soilbox as opposed to on the sides as in our most current model (figure 2).
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Status Report Winter 2013
Figure 4. Soilbox and fluid circulations subsystem (isolated view). Two liquid pumps are attached to their
source tanks--modeled as one object--along the top of the payload assembly. As the centrifuge spins up, an
acceleration field will develop, where the pumps will actually be below their source tanks and near the bottom of the
field. At the very bottom of the field would be the effluent collection tank--designed with an indented space for
accommodating one of the actuators. Water and contaminants from the soilbox will collect in the nine segregated
internal chambers of the effluent tank, each connected to a different part of the soil box (not visible in this view).
Risk Analysis
The project overall and its various components introduce numerous risks in terms of feasibility,
capabilities of the final system, safety and potential damage (table 4). If the mass of the onboard assembly
cannot be lowered, then the highest operational rotational speed of the centrifuge must be reduced,
limiting the variety of experiments that can be conducted by the research team. Exceeding the rated massspeed combination may cause damages to the centrifuge arm, rotor, stator, and pose potentially lifethreatening hazards to nearby personnel if a part were to detach and fly lose with enough energy to
penetrate the centrifuge's outermost shell. In terms of components, the risks can be further categorized
under the areas of external casing, actuator, sampler probe, sample collection, pumps and piping,
moisture and leakage, signal noise, centripetal and Coriolis stress and effects.
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Status Report Winter 2013
Table 4. Risk analysis by likelihood and severity. Severity levels from 'Critical' and up are considered
especially important, whereas likelihood levels below 'Moderate' are given less consideration. The items included are
as follows--mass (M), actuator jam (AJ), actuator life (AL), probe protection (PP), probe flexing (PX), sample
overflow (SO), pump resolution (PR), pump failure (PF), moisture and leakage (ML), signal noise (SN), stresses and
dynamic effects under Coriolis and centrifugal forces (CC).
Likelihood \
\ Severity
Negligible
Slight
Slightly more
Moderate
Slight
Minor
AL
Significant
PF
Critical
PX
PR
AJ
SO
SN, CC, ML,
PP
M
Catastrophic
Since the external casing is one of the heaviest yet most customizable components, it makes sense to make
it the focus of our mass reduction efforts. In the current model, an one-eighth-inch thick cubic shell made
of alloy steel would give a mass of 30 kilograms and a factor of safety of 5 when loaded with every
component under 100 G of acceleration, as simulated by SolidWorks's finite-element analysis software
(figure 5). It may be possible to use even less material by adopting a honeycomb structure, especially on
the walls least loaded. We aim to have a safety factor of at least 3 everywhere. The remainder of the
payload mass comes from the actuator plus motor packages and the amount of water and soil needed by
the research team, all of which are difficult to size-down beyond an estimated additional mass saving of at
most 10 Kg.
8
Status Report Winter 2013
Figure 5. External casing finite-element simulation. Under 100 G of acceleration, the external casing model,
in addition to its own weight, was loaded with distributed and concentrated loads on its faces to simulate the
components inside. The only boundary conditions were its bottom four edges, set as fixed constraints, since they
would be mounted on the centrifuge arm. One of the faces became heavily loaded, approaching a von Mises stress of
120 MPa. The material used was an alloy steel having a yield stress of 600 MPa. All simulation was performed using
SolidWorks's finite-element analysis software.
During operation, the actuators will be affected by their own mass under 100 G of acceleration in a
cantilever fashion with both a shear load and a bending moment (table 5). They must also provide a peak
axial force of 2500 Newtons, as measured in preliminary tests, in order to drive its attached sampler
probe set through less saturated, more finely grained types soils. If these requirements do not exceed the
capacities of any chosen actuator, then the risk of its jamming should be minimal. In addition, most
commercially available models are rated at a life travel expectancy of hundreds of millions of inches. For
the intended purposes in our project, the actuators will move in average 12 inches every week, giving a
usage expectancy in excess of 150 thousand years.
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Status Report Winter 2013
Table 5. Loads on the actuator modeled as a cantilever and its expectancy. Both formulas and numerical
results for a candidate actuator are provided, with the additional information of the axial force requirement obtained
from preliminary testing and measurements, usage data from consultation with the research team, and life travel
expectancy from actual datasheets.
Quantity of interest
Formula
Numerical example
Example capacity
Length
L
0.2 m
n/a
Mass
M
7 Kg
n/a
Weight under 100 G
W ~= 1000 * M
7000 Newtons
n/a
Shear force (lateral load)
Fz = W
7000 Newtons
8000 Newtons
Bending moment
My = W * L / 8
175 Newton-meters
300 Newton-meters
Axial force
n/a
2500 Newtons
3000 Newtons
Usage under given loads
n/a
624 inches per year
160 million inches life
The sampler probes will also be loaded in a cantilever fashion. To prevent deviation from their intended
paths, a plate with a grid of holes matching those on the soilbox will be secured around the probes to
guide them as they move up and down (figure 5). In addition, the probes themselves are soft and can be
easily damaged when being forced through drier soils. A thin, rigid, form-fitting casing will be used to
cover each probe, including its tip (figure 6). A small window is cut in the casing at the distal end of each
probe to allow it to draw water from the soil once it is in position.
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Status Report Winter 2013
Figure 5. Actuator assembly showing attached probe sets and guide plate.
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Status Report Winter 2013
Figure 6. Close-up view of the probe set showing protective casings and cut-out windows for
extracting fluid from soil.
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Status Report Winter 2013
III. Project Management
The project management approach adopted this quarter was to tailor tasks to each member's strengths
(table 6). Progress has been difficult, however, in part because of struggles with basic as well as new
engineering concepts encountered during the research and design phases. Turn-around time has been
slow, with some action items incomplete after more than one month. A better strategy for the upcoming
quarter may be to reduce the total number of tasks, since even though a few research and explorative
objectives still remain, we now know a lot more specifically about what need to be and can be
accomplished. Possible focus areas with definable expected outcomes include mass reduction of the
external casing, stress and material analysis of fastener and mounting options, communicating with
vendors regarding actual actuator capabilities and limitations, stress and material analysis of soilbox
models, Coriolis effect and centripetal stress calculations, vibration and harmonic analysis of components,
finalizing dispersion cap and collection tank CAD models, design of floating inlet tubes for liquid pumps,
suction tests using different vacuum levels and on different types of soils, pressure drop tests of
hydrophobic barrier materials, selecting or designing an electrical box for powering different components,
hardening of electronic components, coding for wireless communication via Arduino and XBee,
controlling devices and interpreting signals through LabView and its Arduino toolkit, and building a
LabView graphical user interface. Some of them require additional parts and materials to be purchased,
but most can be worked on while waiting for our orders to arrive. Thus, each member will know his
minimum responsibilities for the entire quarter right from the beginning. Simultaneously, as suggested by
our advisors, team members may be required to prepare a presentation and a progress report, perhaps
once every two weeks, in order to better track progress and difficulties, encourage understanding of what
they are doing as related to the project as a whole, and consequently be better prepared for the quarterend final presentation and report.
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Status Report Winter 2013
Table 6. Project work breakdown schedule for Winter quarter 2013. Overall descriptions of responsibilities
tend to be general, since much of the specific component requirements and layouts were still not figured out at the
beginning of the quarter. Many new research, design, and analysis goals were found to be necessary through the
course of this quarter.
In terms of project timeline, even by the most optimistic projections, a full critical design review must be
postponed until the end of the upcoming quarter, most likely in May. More likely, however, only most of
the designs will be ready, not every single component. Meanwhile, some additional testing on certain
designs and components should be able to be completed in the upcoming quarter. We aim to have a
substantial amount of the designs and preliminary testing required by this project ready for review as a
foundation for any future work.
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Status Report Winter 2013
IV. Summary
Going from last quarter, a good amount of design, analysis, calculations, and component selection have
been accomplished, not all of which have been included in this report. However, two important
observations still remain valid, namely the large scope of this project with its myriad components and
technical challenges and the struggles this team has in terms of catching up on basic engineering
knowledge and skills, learning new materials encountered throughout the course of this project, and
maintaining a consistent, acceptable turn-around time on assignments.
15